<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <div><input type="text" value="">
        <button>判断是否是两位数</button>
    </div>
</body>

</html>
<style>
    div {
        width: 400px;
        height: 40px;
        margin: 0 auto;
    }
</style>
<script>
    var ipt = document.querySelector("input")
    var but = document.querySelector("button")
    but.onclick = function () {
        var len = ipt.value;
        var n = len.length;
        alert(n == 2 ? "✔是两位数" : "这是" + n + "位数");
    }
</script>